The Project 64 etext for Commodore diagnostics, converted by The
Basic Bombardier. This etext was created by concatenating several
documents into one. Please see the header of the particular document
for original sources.

Disclaimer: I hereby disclaim any responsibility for the accuracy of
the information on this chart and disclaim all liability for the use
thereof. This chart is supplied as an adjunct to Commodore computer
users. It is intended for assistance in isolating a faulty chip and
is to be used by persons experienced in the repair of computers and
computer related devices.

Printed in Australia – 1986. No path of the Commodore Diagnostician
may be reproduced nor copied for any purpose without the author’s
approval. (c) Copyright.

> I have a 1541 that just spins forever when turned on. Both LED’s
> are on. Anyone have any pointers to repair info on this model?

There are several things that can cause this, but the most common is a
bad 901229-05 ROM. The 6502, 6522, RAM, and a couple of the TTL chips
in the reset circuit can also cause it, pretty much in that order of
likelyhood, assuming it just „went bad“ and didn’t fail because of
something you did to it.

> Well, I’ve done it again. I bought another 1541 drive that doesn’t
> work. The guy told me it was „out of alignment“. Needless to say, that
> was an understatement. So, I need some help trying to fix it.
>
> Here are the symptoms. When I power it on, the red and green LED’s
> both flicker on for about 1/4 of a second, then stay off. The drive
> motor spins constantly when powered on, and will not respond to
> commands. Do you why that is?

The power supply is probably failing. Note that the power indicator is
going out under load. Since the +12 volts runs the motors, try the +5v
line… bet you’ll find a bad bridge rectifier or flaky regulator.

In article <4p3vbp$s7i@dub-news-svc-4.compuserve.com> you write:
>Well, I’ve done it again. I bought another 1541 drive that doesn’t
>work. The guy told me it was „out of alignment“. Needless to say, that
>was an understatement. So, I need some help trying to fix it.
>
>Here are the symptoms. When I power it on, the red and green LED’s
>both flicker on for about 1/4 of a second, then stay off. The drive
>motor spins constantly when powered on, and will not respond to
>commands. Do you why that is?
>
>Here’s what I’ve tried to find out where the problem is. I swapped the
>main processor board with a working drive, and it seemed to power up
>OK. So I put the original, faulty board back in and tried swapping out
>chips. I replaced the 901229-05, 6502, both 6522’s, and the tiny
>EL7407-0284 chip with no posititve effect. No other chips in this
>drive have the chip sockets for easy replacement.
>
>901229-05 6502
> 6522 EL407-0284
> 6522
>
>If you know what the problem is or have some suggestions on what I
>could do to diagnose it, PLEASE! Thanks.
>
>——-
>Cris „PC-Geek“ Berneburg <74171.2136@compuserve.com>
>The Basic Bombardier, Manager of Project 64
>http://ourworld.compuserve.com/homepages/pcgeek/
>PGP public key A1CE4355 available on keyservers
>

The LED flicker tells you immediately that there is a problem with
the power — they aren’t connected to anything else. Most likely
you have a blown rectifier. There are two of them, at the back
of the drive, and they look like this:

_____
/ |
/ |
| |
-------
| | | |

That is, a notched square with four pins coming out of the bottom.
After you power up the drive, at least one of them (the smaller one)
will probably be hotter than blazes.

The fix: go to Radio Shack and buy a new, heftier one for $1 or so,
and solder it in. These are full-wave rectifiers of course.

Blank screen is the most common symptom, and a failing PLA chip is the most common reason. However, quite a few other failures can cause it as well, such as a bad power supply (check with a known good substitute), bad RAM chip(s), and in general, just about any other chip in there because many chips share multiple data lines. If any one of those lines is loaded down or missing a signal for whatever reason, it can produce that symptom. To narrow it down a bit:

Turn the computer off and back on rapidly about five times. If the screen ever comes up with flashing colors or all one color, the PLA is suspect. Replace it to check. Try a cartridge, such as a game. It essentially „replaces“ some of the chips in the computer when it runs. If a cart works, check the ROMs. The screen may have a normal border even if the CHARacter or BASIC ROMs are bad. A bad Kernal ROM will produce no border.

The internal RF unit outputs a signal that goes to the antenna input of your TV. If the picture is snowy, suspect the RF modulator, assuming the direct video output of the 64 is normal. If the computer is „dead“ but is getting power (red LED on), the modulator will produce a black screen… darker than the blank screen of a failing chip in the computer. A missing 9VAC (power supply problem) is a possiblility. Note that the later C64C will still work without the 9VAC or if the internal fuse opens. However, you will have no sound, the cassette will not work, and the TOD clocks will not work.

See if any of the RAM chips (there are eight of them) get warm or hot… feel each one with the back of your finger after the computer has run for about 5 minutes. Shorted chips will get hotter than the others. Note: bad RAM doesn’t always get hot. See if the computer resets the other components in the system like the drive and/or printer. If so, try a „blind“ disk command and see if the drive responds.. try formatting a disk. If that works, you may have a bad VIC chip (no screen display). Sometimes a bad SID chip will produce a blank screen… pull it out and try the computer. It will run without it, although you will have no sound, and a proportional mouse will not work. The few large chips that normally run hot have a high failure rate: in rough order… the PLA, SID, MPU and the VIC. Static zaps usually take out chips like the CIAs. A shorted CIA can produce a blank screen. Note that you will get the startup screen with the CIA’s removed. You can use that as a diagnostic. The smaller so-called „glue logic“ chips (TTL) run cool and are pretty rugged. Although they rarely fail, I’ve had a few that drove me crazy by making me unsolder a dozen IC’s until I found the bad one. With the above noted exceptions, removing a chip will not allow the computer to produce the startup screen. Removing a chip will in most cases produce a blank screen.

The only practical way to check chips is by substitution. The easiest way to do that is by inserting each suspected chip into a working computer that already has all chips socketed. (You can chase your tail doing it the other way around if you have more than one bad one.) I made a test board for just that purpose. Suspected chip(s) can be tested one at a time and only the bad ones need be replaced. At the very least, you need a source of known good chips for test purposes. Be careful… they are static sensitive. If you don’t want to go to that much trouble to diagnose the problem, you will probably be better off hunting up another C64. Chips are hard to find and expensive. Keep a spare „breadbox“ or two, even if only for parts.

1541 CHIPS VS SYMPTOMS latest additions or corrections: 2-15-08
This list represents the most common versions of the 1541 in the standard brown case with PC board numbers 1540050 (early ALPSpush-down door mechanism) and the 251830 and 251777 (Newtronics twist door mechanism). Although the very early „long board“ (white case) 1541 is not represented here, major chip functions are of course similar. Thatdrive used more TTL (so-called „glue logic“) chips that were later „integrated“ into a single motor control IC. Although functionally identical, newer drives such as the 1541C and 1541-II integrate more functions into fewer more specialized IC’s, making some repair parts even harder to find. Although most of the chips in the different versions of the standard brown case 1541 are the same, board layout and some chip ID numbers may be different. To eliminate confusion, I will list the two major versions of the drive separately. In another article (fix1541.txt) I will give you some troubleshooting tips. Included is how the drive should behave during normal operation and what is likely to cause a particular fault. Before suspecting any IC chips, don’t overlook more common causes of problems such as a dirty read/write head. Always check the „easy stuff“ first.

When drive powered up, red LED stays on and spindle motor runs continuously. DOS ROM is a common failure. Check also UA1, UC2, UC4, UC5, UC6, UD3, and UD5.

UC1 325572-01 MOTOR CONTROLLER

When drive powered up, red LED comes on and goes out, but spindle motor does not turn. When LOAD attempted, spindle does not turn, red LED flickers, screen displays „FILE NOT FOUND“ and red LED flashes.

When drive powered up or reset, spindle motor runs momentarily, but red LED doesn’t come on. When LOAD attempted, screen indicates „SEARCHING FOR …“ but red LED does not light, spindle runs continuously, and screen shows „FILE NOT FOUND“ error.

UD2 7417 LOGIC (R/W CONTROL BUFFER)

When drive powered up or reset, red LED comes on and goes out, but spindle motor does not turn. If LOAD is attempted, red LED comes on, stepper moves slightly, spindle doesn’t turn, screen shows „FILE NOT FOUND“ and red LED flashes.

Powers up normally. When LOAD attempted, spindle turns and red LED comes on, but „FILE NOT FOUND“ and red LED flashes. Check also UF3 and UF4.

UE6 74LS193 (7417) LOGIC (BIN COUNTER)

Drive powers up and resets normally, but if LOAD is attempted, spindle motor runs with red LED out. Screen displays „SEARCHING FOR …“ and stepper does not move. Check also UD4.

UF3 NE592N READ PREAMPLIFIER

Powers up normally. When LOAD attempted, spindle runs and red LED comes on, but „FILE NOT FOUND“ and red LED flashes. Check also UE4 and UF4.

UF4 NE592N READ AMPLIFIER

Powers up normally. When LOAD attempted, spindle runs and red LED comes on, but „FILE NOT FOUND“ and red LED flashes. Check also UE4 and UF3.

VR1 UA7812KC (LM340KC-12) +12 VOLT REGULATOR

Drive powers up „normally“, but motors run slowly or not at all. If that happens, drive access will produce a flashing red activity LED and errors „file not found“ or „drive not ready“ on the error channel.

VR2 UA7805KC (LM340KC-5.0) +5 VOLT REGULATOR

Green (power) LED dim, flickers, or does not come on at all, but spindle may run continuously and red LED may be dark.

CR1 2 AMP 200V BRIDGE RECTIFIER (FOR +12V)

Drive appears to power up normally, but motors do not run. Should get warm only if the drive is being accessed (motors running).

CR3 2 AMP 200V BRIDGE RECTIFIER (FOR +5V)

On power up, green and red LED’s are dim or dark and spindle motor runs continuously. May be intermittant and „die“ after warmup. Part runs very hot normally and is a common failure in this model.

Partial failure common cause of „SEARCHING FOR“ (check also UC3) and „DEVICE NOT PRESENT“ errors. Total failure: drive powers up OK, but does not respond to computer… no reset or disk access.

UB2 TMM2016AP-10 16K RAM TMM2116AP-15 or MB8128-15

When drive powered up, spindle motor runs continuously and red LED flashes about once every two seconds.

UB3 325302-01 DOS ROM

When drive powered up, red LED flashes 3 times repeatedly.

UB4 901229-03 or -05 DOS ROM

When drive powered up, red LED stays on and spindle motor runs continuously. DOS ROM is a common failure. Check also UA1, UC2, UC4, UC6, UC7, UD3, and UD5.

UC1 325572-01 MOTOR CONTROLLER

When drive powered up or reset, red LED comes on and goes out, but spindle motor does not turn. When LOAD attempted, spindle does not turn, red LED flickers, screen displays „FILE NOT FOUND“ and red LED flashes.

When drive powered up, spindle motor runs continuously. Red LED may stay on, or flash three times and go out.

UD1 7406 (M53206P) LOGIC

When powered up, spindle motor runs momentarily, but red LED doesn’t come on. When LOAD attempted, screen indicates „SEARCHING FOR …“ but red LED does not light and spindle runs continuously.

UD2 7407 (7417) LOGIC (R/W CONTROL BUFFER)

When drive is powered up or reset from computer, red LED comes on and goes out, but spindle motor does not turn. When LOAD is attempted, stepper moves slightly, spindle doesn’t turn, and error message on screen is „FILE NOT FOUND“ with flashing red LED.

Drive powers up and resets normally. Spindle motor runs, stepper moves slightly, but „FILE NOT FOUND“ error, and red LED flashes. Check also UF3 and UF4.

UE6 74LS193 (7417 or 7407) LOGIC (COUNTER)

Drive powers up and resets normally, but when LOAD is attempted, screen indicates „SEARCHING FOR …“ red LED does not light and spindle runs continuously. Check also UD4.

UF3 NE592N (LM592) READ PREAMP

Drive powers up and resets normally. Spindle motor runs, stepper moves slightly, but „FILE NOT FOUND“ error and red LED flashes. Check also UF4 and UE4.

UF4 NE592N (LM592) READ AMPLIFIER

Drive powers up and resets normally. Spindle motor runs, stepper moves slightly, but „FILE NOT FOUND“ error and red LED flashes. Check also UF3 and UE4.

VR1 UA7812KC (LM340KC-12) +12 VOLT REGULATOR

When powered up, green power LED comes on and red LED comes on and goes out normally, but motors do not run (or move slowly). Drive access will produce a flashing red activity LED and error messages „file not found“ or „drive not ready“ on the error channel.

VR2 UA7805KC (LM340KC-5.0) +5 VOLT REGULATOR

Green (power) LED dim, flickers, or does not come on at all. Drive appears dead, but spindle motor runs continuously.

CR1 2 AMP 200V BRIDGE RECTIFIER (FOR +5V)

Drive appears dead, but spindle motor runs continuously with both green and red LEDs dim or dark. May be intermittant and fail after warmup. This part normally runs hot and is a common failure item.

CR3 2 AMP 200V BRIDGE RECTIFIER (FOR +12V)

Drive appears to power up normally, but motors do not run. Should get warm only if drive is accessed (motors running).

Notes: UC2, UC3, UB4, and regulators VR1 and VR2 run warm normally and bridge rectifiers CR1 and CR3 run very hot if drive is being accessed. Otherwise, only CR3 will get hot.